Power semiconductor devices, such as power MOS (metal-oxide-semiconductor) transistors or power diodes, include a drift region and a pn junction between the drift region and a further device region, such as a body region in an MOS transistor and an emitter region in a diode. The doping concentration of the drift region is lower than the doping concentration of the further device region, so that a depletion region (space charge region) mainly expands in the drift region when the device blocks, which is when the pn junction is reverse biased.
The dimension (length) of the drift region in a current flow direction of the device and the doping concentration of the drift region mainly define the voltage blocking capability of the semiconductor device. In a unipolar device, such as a power MOSFET, the doping concentration of the drift region also defines the on-resistance of the device, which is the electrical resistance of the semiconductor device in the on-state.
When the pn junction is reverse biased dopant atoms are ionized on both sides of the pn junction resulting in a space charge region that is associated with an electrical field. The integral of the magnitude of the field strength of the electrical field corresponds to the voltage that reverse biases the pn junction, whereas the maximum of the electrical field is at the pn junction. An Avalanche breakthrough occurs when the maximum of the electrical field reaches a critical field strength that is dependent on the type of semiconductor material used to implement the drift region.
The doping concentration of the drift region may be increased without reducing the voltage blocking capability of the device when charges are provided in the drift region that may act as counter charges to ionized dopant atoms in the drift region when the pn junction is reverse biased.
According to a known concept, at least one field electrode or field plate is provided in the drift region, is dielectrically insulated from the drift region by a field electrode dielectric, and may provide the required counter charges. The field electrode can be connected to one of the load terminals of the semiconductor device, such as the source terminal in an MOS transistor or the anode terminal in a diode. A voltage across the field electrode dielectric is dependent on the magnitude of the voltage that is applied between the load terminals and that reverse biases the pn junction and on the length of the field electrode in the current flow direction. Depending on the voltage blocking capability of the semiconductor device, the voltage across the field electrode can be up to several 100V, so that a thickness of the field electrode dielectric of up to several micrometers (μm) is required. In a semiconductor device having a voltage blocking capability of 300V, the required thickness of the field electrode dielectric is, for example, between 3 μm and 4 μm.
In vertical power semiconductor devices the field electrode and the field electrode dielectric are arranged in a trench. The field electrode and the field electrode dielectric can be formed by oxidizing sidewalls of trench, so as to form an oxide layer as field electrode dielectric, and by filling a residual trench with a field electrode material. Thick vertically extending oxide layers that are able to withstand high voltages, however, induce mechanical stress that may cause defects or damages of the semiconductor body.
There is therefore a need to provide a method for producing a semiconductor device with a thick vertically extending dielectric layer.